1. Field of the Invention
This invention relates to apparatus for accurately and remotely determining the temperature and emissivity of a body.
2. Brief Description of the Prior Art
The temperature and emissivity of a body is determined by measurement of the radiation, called the spectral radiance, which is emitted by the body. The spectral radiance W(.lambda.) of a body of temperature T and emissivity (.epsilon.) is given by Planck's Law as: ##EQU1## where W(.lambda.)=spectral radiant emittance, W cm.sup.-2 .mu..sup.-1
.lambda.=wavelength, .mu. PA1 T=absolute temperature, degrees K. PA1 C.sub.1 =(3.3.7415 +/- 0.003).times.10.sup.4 W cm. PA1 C.sub.2 =(1.43879 +/- 0.00019).times.10.sup.4 degrees K. PA1 .eta.=detector quantum efficiency (electrons/photon) PA1 To=object temperature .epsilon..sub.o =emissivity of the object. PA1 A.sub.D =area of the detector PA1 (.lambda..sub.Li, .lambda..sub.ui)=spectral limits of the ith sub-band PA1 .tau..sub.A (.lambda.)=atmospheric transmission PA1 .eta.(.lambda.)=detector quantum efficiency PA1 .eta..sub.i =noise present in the estimate PA1 J.sub.ij =(i,j) term of Fishers Matrix PA1 E(.)=expected value operation PA1 1n(.)=natural logarithm operation PA1 Ri=sensor measurements in the ith sub-band PA1 A=estimate of A.
The emissivity factor can also be a function of wavelength, i.e., .epsilon.=.epsilon.(.lambda.). If contact can be made with a radiating body, the temperature can be readily measured with a thermocouple. If the composition of the surface of the body is known, emissivity can be estimated. However, if the body is remotely located with respect to the instrumentation, temperature and emissivity estimates must be derived from the only quantity available to the experimenter, namely, the radiation emitted by the body. As an example, it may be desirable to make temperature and emissivity measurements of a rocket plume. The environment of the plume may preclude placing instrumentation in the plume or the rocket could be in flight whereby the plume would be inaccessible. A remote temperature and emissivity measuring device in this instance would be useful.
The passive radiation, W(.lambda.), available to the instrument will range from a lower wavelength cutoff .lambda..sub.L to an upper wavelength cutoff .lambda..sub.u. Wavelengths lower than .lambda..sub.L and higher than .lambda..sub.u are not available for several reasons. The amount of radiation of the body outside these limits may be very small or non-existent. Detectors sensitive to the radiation may not exist or be prohibitively expensive. The atmosphere between the instruments and the body may absorb or scatter the radiation before it reaches the instrument.
An instrument to measure temperature and emissivity of an object from its spectral radiance is depicted in FIG. 1. An afocal lens recollimates the incoming radiation and directs it to an imaging lens. The imaging lens creates an image of the object in the focal plane. A detector sensitive to wavelengths from .lambda..sub.L to .lambda..sub.u is placed in the focal plane. A wheel with N filters is positioned so that one filter at a time is placed in the optical path. Each filter i transmits radiation for .lambda..sub.iL to .lambda..sub.iu to the detector. The sub-band (.lambda..sub.iL, .lambda..sub.iu) is contained in the pass band (.lambda..sub.L, .lambda..sub.u). The wheel is moved into several positions, allowing the detector to make measurements in each of the N sub-bands. The output of the detector is amplified, digitized and sent to a processor to determine temperature and emissivity. An alternate configuration is shown in FIG. 2 wherein a separate detector is available for each sub-band. Each detector has a filter in front of it allowing only radiation in the desired sub-band to reach the detector. A scanner moves the image by the detectors which successively gathers radiation from the image. The processor then takes the output of the detectors and determines the temperature and emissivity estimate of the object.
Remote temperature estimating devices of the type shown in FIGS. 1 and 2 fall into several categories. In the first category, the spectral radiance is measured in one preselected wavelength band and the magnitude of the radiation is related to the object temperature by a table look up processor. More specifically, radiation from the object whose temperature is to be measured enters the lens and is imaged into the detector. A filter placed in front of the detector restricts the radiation falling on the detector to the preselected band from .lambda..sub.1 to .lambda..sub.2. Output R.sub.T is given by: ##EQU2## where A.sub.L =area for the lens
The processor contains candidate values of R.sub.T computed beforehand for a wide range of temperatures. The emissivity is generally assumed to be unity and atmospheric transmission is also assumed to be unity. If these assumptions are correct, the instrument will yield a reasonable answer. However, under many conditions, these assumptions are not valid and the temperature estimate will be inaccurate.
In the second category, the "color ratio" approach is used. The measurement of the radiation in band 1 and band 2, denoted by R1 and R2, respectively, are used to form the ratio, ##EQU3## This ratio can then be used to form a temperature estimate T as follows: EQU T=f(z). (4)
The relationship of f(z) can be derived from Planck's law. If the measurements R1 and R2 are low in noise, T can be an accurate estimate of temperature. However, noise is a reality in many applications and noise is significant. There is no obvious way to incorporate the data from other spectral bands into the ratio. Hence, the color ratio method does not make use of available data.
The measurements from several wavelength bands may be available for temperature estimation. The sensor can be a single detector device with a plurality of spectral filters successively laced in front of the detector (FIG. 1) or a multiple detector device with each detector preceded by a single spectral filter (FIG. 2). The instrument can also be a Fourier spectrometer. The resulting measurements are then curve fit to the black body spectral distribution as given by Planck's law (Equations (1) and (2). The temperature which provides the best fitting curve to the data is taken as the temperature of the object. This procedure neglects to make use of the fact that some spectral bands are more useful than others in estimating temperature. In addition, there is no obvious way to incorporate a priori data which may be available.